Stress in Teeth Caused by Amalgam Expansion

Objectives: Clinicians incidentally encounter tooth cracks emanating from amalgam restorations. A companion study showed restoration expansion and outward cuspal flexure for molars with amalgam restorations after storage in saline. In this study amalgam expansion was measured and then applied in a finite element analysis to investigate associated tooth stresses.
Methods: Eleven hemispheres (approximately 5mm circumference and 2mm high) of zinc-containing amalgam (Permite, SDI) were immersed in saline solution for 8 weeks. Images were captured before and during storage with a stereomicroscope. Sample outlines were traced to determine volumetric changes (ΔV) from projected surface area changes (ΔV≈3/2 ΔA). ANOVA and Student-Newman-Keuls post hoc were used to test the significance of the volume changes (α=0.05). The volumetric changes were subsequently applied in cross-sectional finite element models (MARC, MSC Software) of a restored molar (cavity depth 3.5mm, width 3.5mm). Three cavity shapes with increasing wall convergence (0-10 degrees) were analyzed.
Results: The table shows the experimental volumetric expansion (mean ± standard deviation). Mean volumetric expansion increased during the 8 weeks, although differences between 2-, 4-, and 8-week values were not statistically significant. Applying the expansion in the finite element analysis caused outward cuspal flexure, resulting in stress concentrations at the cavity floor and line angles. This contrasts with stress distributions caused by shrinking composite restorations, where significant stresses in the tooth are usually found on the external buccal and lingual surfaces. The 5 to 10-degree wall convergence increased cuspal flexure 18-19% compared to a cavity with straight walls.
Conclusions: Volumetric expansion was measured for amalgam over a period of 8 weeks. Cavity walls were pushed outward by amalgam expansion, which caused stresses at the bottom of the cavity. These stresses may contribute to initiation of the tooth cracks that are incidentally observed clinically around amalgam restorations.